1. Field of the Invention
This invention is related in general to the field of metrology of optical lenses and electro-optic systems. In particular, the invention relates to high-throughput measurements of the optical transfer function of lenses and imaging systems.
2. Description of the Related Art
For the purposes of this disclosure, the term “optics” is used to designate both the field of optics and optical components or systems, as is commonplace in the art. In the field of optics, where objects are imaged by a lens or a system of lenses and the resulting images are used for further processing, it is very important that the images produced by the optics be as true as possible to the objects from which they are formed. The accuracy with which optics produce the image of an object is characterized in the art by the so-called optical transfer function (OTF). The OTF is a combination of the optics' resolution performance, measured by its phase transfer function (ΦTF), and of its ability to transfer contrast, measured by the modulation transfer function (MTF).
Thus, the MTF of an optical system is a measure of its transfer of contrast from the object to the image at a given spatial frequency. As the object details get smaller (i.e., the spatial frequency of the signal increases) the optics' ability to transfer contrast is reduced. In essence, the MTF is a quantitative assessment of how well the optics maintain contrast level as a function of frequency. The MTF of a lens or system is typically represented by a plot of contrast (modulation) as a function of spatial frequency for each field point as measured at the detector plane.
The phase transfer function ΦTF, on the other hand, indicates how much the detail (frequency information) of an object shifts in position at the image plane relative to the object plane at each spatial frequency. Although an ideal ΦTF would be equal to zero at all frequencies, in fact distortion tends to introduce a linear term which may be important in characterizing the performance of an imaging system. Nonetheless, commercial applications often specify image quality only in terms of the system's modulation transfer function. This practice probably stems from the fact that ΦTF's non-linearity has the most influence at high spatial frequencies, where contrast is quite low. Therefore, since low-contrast conditions are typically not of interest, the impact of phase shifts is less significant under practical working conditions, where contrast must be high.
Within the optics industry, it has been normal practice to calculate the OTF of an imaging system using a variety of illumination sources and targets, such as the pinhole, the slit, and the knife edge. The resolution and contrast performance is measured by how well the imaging system reproduces, for example, a sharp edge illuminated by a light source. The approach is simplified by the fact that, as is well understood in the art, the MTF corresponds to the modulus of the one-dimensional Fourier Transform of the line spread function of the imaging system. The line spread function in turn is the derivative of the edge spread function, which describes the image of a line displayed by the detector on the detector entrance face. Therefore, the MTF is readily obtainable from the edge trace formed on the detector by an object with a sharp edge, such as the straight-line edge of a knife.
As illustrated in FIG. 1, the discrete nature of the sensing elements of a pixellated detector necessarily causes the signal to be approximated. The edge trace of the signal received across a sequence of pixels 10 along the edge-response detection line, which in reality decreases progressively with an approximately S-shaped functionality from maximum intensity to zero (shown on the right side of the figure), is reflected at each pixel by intermediate discrete values between a maximum and a minimum (on the left of the figure). Correspondingly, the slope of the edge trace (the line spread function) is characterized by discrete derivative values, rather than by a continuous bell-shaped curve with a single maximum (as shown on the right side of the figure).
One of the problems consistently encountered in this type of measurement is the fact that sampling is often insufficient (that is, the detector is operated below its Nyquist limit of sampling), which produces aliasing in the imaged signal. The traditional method of circumventing this problem has been to use a microscope objective to magnify the region of interest, thereby providing sufficient sampling for operation within the detector's Nyquist limit. FIG. 2 illustrates a typical set up wherein a microscope objective 12 is placed between the test lens 14 and the detector 16 to relay an image of the target 18. A conventional knife edge is positioned on the target 18 normal to the edge-response detection line and a scanning mechanism (illustrated by the bi-directional arrows in the figure) for the objective is used to cover the entire field of view of the optics being tested.
While effective, this solution has the drawback of narrowing the field of view of the detector, which therefore requires that a scanning mechanism be utilized to span the field of the optics being tested. This, in turn, introduces alignment and data-stitching challenges to the process of data acquisition, in addition to the cost of the additional optics and scanning hardware. Most importantly, the use of a microscope objective and the attendant scanning operation greatly affect the speed with which the optical transfer function of an imaging system can be measured.
Current trends in the optics industry require high-precision manufacturing of optical elements and related quality-control processes that are easily adaptable for large-volume production. While production facilities have been able to readily scale up for volume, the quality-control functions have not been able to keep pace with increased manufacturing capabilities. Commercially available instruments for quality control are designed for expensive and relatively slow testing procedures for cost-insensitive low-volume projects. Accordingly, efforts in the art have been directed to improving the quality of the measurements, rather than the speed and suitability of the tests for high-volume quality-control purposes. U.S. Pat. Nos. 4,241,996, 4,972,451 and 5,748,230 describe various techniques based on this general objective of improving accuracy.
Therefore, there is still a strong need for a high-throughput approach for testing optics produced in a high-volume production environment. The ISO international standard for measuring the spatial frequency response (SFR) of electronic still-picture cameras describes a system wherein a slanted edge is used to eliminate the effects of aliasing. The present invention is based on extending this concept to the process of testing imaging optics with multiple, simultaneously imaged optical targets.